1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a method for crystallizing silicon in both the X-axis and Y-axis directions without a rotation of a stage.
2. Discussion of the Related Art
As information technologies develop, various displays are in demand. Recently, many efforts have been made to research and develop various flat display panels such as a liquid crystal display (LCD), a plasma display panel (PDP), an electroluminescent display (ELD), a vacuum fluorescent display (VFD), and the like. Some types of the flat display panels have already been applied to various display devices.
LCD devices are widely used because of their characteristics and advantages, including high quality images, light-weight, thin and compact size, and low power consumption so as to be used in place of cathode ray tubes (CRT) for mobile image display devices. An LCD device has also been developed to receive broadcast signals to display, such as a television, a computer monitor, and the like.
Even if there are significant developments in LCD the technology for an image display in various fields, the image quality fails to meet the characteristics and advantages of an LCD display. In order to use a liquid crystal display device as a display device in various applications, development of an LCD device depends on realizing high image qualities, such as high resolution, high brightness, wide screen, and the like, as well as maintaining the characteristics of lightweight, compactness, and low power consumption.
An LCD display device includes an LCD panel displaying an image and a driving unit that applies driving signals to the LCD panel. The LCD panel includes first and second glass substrates bonded to each other so as to secure a space therebetween, and a liquid crystal layer is injected between the first and second glass substrates.
Formed on the first glass substrate (TFT array substrate) are a plurality of gate lines arranged in one direction at fixed intervals a plurality of data lines arranged in a direction substantially perpendicular to the gate lines at fixed intervals, a plurality of pixel electrodes in pixel areas defined by the gate and data lines crossing each other, and a plurality of thin film transistors switched by signals on the gate lines to transfer signals of the data lines to the pixel electrodes. Formed on the second glass substrate (color filter substrate) are a black matrix layer that shields light in areas outside the pixel areas, an R/G/B color filter layer for realizing colors, and a common electrode for producing an image.
The above-described first and second substrates are separated from each other by spacers to provide a space therebetween, and are bonded to each other through a sealant having a liquid crystal injection inlet. Further, liquid crystal is injected between the two substrates. At this time, the liquid crystal injection method is carried out by maintaining a vacuum state within the gap between the two substrates bonded by the sealant. Then, the liquid crystal injection inlet is dipped in a vessel containing liquid crystal, so as to allow the liquid crystal to be injected into the gap between the two substrates by osmosis. After the liquid crystal is injected as described above, the liquid crystal injection inlet is sealed with an air-tight sealant.
The operating principle of a general liquid crystal display device uses the optical anisotropy and polarization characteristic of liquid crystal. Because of the thin and long structure of liquid crystal, the liquid crystal molecules are aligned to have a specific direction. Also, the direction of the alignment may be controlled by applying an induced electric field to the liquid crystal molecules. Therefore, when the alignment of the liquid crystal molecules is arbitrarily controlled, the alignment of the liquid crystal molecules is eventually altered. Subsequently, due to the optical anisotropy of liquid crystal, light rays are refracted in the direction of the alignment of the liquid crystal molecules, thereby producing image information.
Among current technologies, the active matrix liquid crystal display (LCD), which is formed of a thin film transistor and pixel electrodes aligned in a matrix and connected to the thin film transistor, is considered to be excellent for its high resolution and its ability to represent moving images.
A poly-silicon, having a high electric field mobility and low photocurrent, may be used to form a semiconductor layer of the above-described thin film transistor. The method for fabricating a poly-silicon can be divided into a low temperature fabrication process and a high temperature fabrication process depending upon the fabrication temperature.
The high temperature fabrication process requires a temperature condition approximate to 1000° C., which is equal to or higher than the temperature for modifying substrates. Accordingly, because glass substrates have poor heat-resistance, expensive quartz substrates having excellent heat-resistance should be used. And, when fabricating a poly-silicon thin film by using the high temperature fabrication process, deficient crystallization may occur due to high surface roughness and fine crystal grains, thereby resulting in deficient device application, as compared to the poly-silicon formed by the low temperature fabrication process. Therefore, technologies for crystallizing amorphous silicon, which can be vapor-deposited at a low temperature, to form a poly-silicon are being researched and developed.
The low temperature fabrication process can be categorized into laser crystallizing and metal induced crystallization processes. The laser crystallizing process includes irradiating a pulsed laser beam on a substrate. By using the pulsed laser beam, the solidification and condensation of the substrate can be repeated at a cycle unit of about 10 to 102 nanoseconds. This low temperature fabrication process is known for having the advantage that the damage caused on a lower insulating substrate may be minimized.
The related art silicon crystallization method using laser crystallizing will now be explained in detail. FIG. 1 illustrates a graph showing sizes of amorphous silicon particles by laser energy density. As shown in FIG. 1, the crystallization characteristic of the amorphous silicon may be described as having a first region, a second region, and a third region depending upon the intensity of the laser energy.
The first region is a partial melting region, whereby the intensity of the laser energy irradiated on an amorphous silicon layer melts only the amorphous silicon layer. After the irradiation, the surface of the amorphous silicon layer is partially melted in the first region, whereby small crystal grains are formed on the surface of the amorphous silicon layer after a solidification process.
The second region is a near-to-complete melting region, whereby the intensity of the laser energy, being higher than that of the first region, almost completely melts the amorphous silicon. After the melting, the remaining nuclei are used as seeds for a crystal growth, thereby forming crystal particles with an increased crystal growth as compared to the first region. However, the crystal particles formed in the second region are not uniform. Also, the second region is narrower than the first region.
The third region is a complete melting region, whereby laser energy with increased intensity, as compared to that of the second region, is irradiated to completely melt the amorphous silicon layer. After the complete melting of the amorphous silicon layer, a solidification process is carried out, so as to allow a homogenous nucleation, thereby forming a crystal silicon layer formed of fine and uniform crystal particles.
In the method for fabricating poly-silicon, the number of laser beam irradiations and degree of overlap are controlled in order to form uniform, large and rough crystal particles by using the energy density of the second region. However, the interfaces between a plurality of poly-silicon crystal particles act as obstacles for the electric current flow, thereby decreasing the reliability of the thin film transistor device. In addition, collisions between electrons may occur within the plurality of crystal particles causing damage to the insulating layer due to the collision current and deterioration, thereby resulting in a performance degradation. In order to resolve such problems when using a method for fabricating a poly-silicon by sequential lateral solidification (SLS) crystallization, the crystal growth of the silicon crystal particle occurs at a surface interface between liquid silicon and solid silicon in a direction perpendicular to the surface interface. The SLS crystallization method is disclosed in detail by Robert S. Sposilli, M. A. Crowder, and James S. Im, Mat. Res. Soc. Symp. Proc. Vol. 452, pp. 956-957, 1997.
In the SLS crystallization method, the amount of laser energy, the irradiation range of the laser beam, and the translation distance are controlled, so as to allow a predetermined length of lateral growth of the silicon crystal particle, thereby crystallizing the amorphous silicon into a single crystal. The irradiation device used in the SLS crystallization method concentrates the laser beam to a small and narrow region, and so the amorphous silicon layer deposited on the substrate cannot be simultaneously changed into polycrystalline. Therefore, in order to change the irradiation position on the substrate, the substrate having the amorphous silicon layer deposited thereon is mounted on a stage. Then, after an irradiation in a predetermined area, the substrate is translated so as to allow an irradiation to be performed on another area, thereby carrying out the irradiation process on the entire surface of the substrate. Alternatively, the laser may be mounted on a stage allowing it to move relative to the substrate.
FIG. 2 is a schematic view illustrating a general SLS (Sequential Lateral Solidification) device. Referring to FIG. 2, the general SLS device includes a laser beam generator 1 for generating a laser beam, a focusing lens 2 for focusing the laser beam generated from the laser beam generator 1, a mask 3 for irradiating the laser beam to a substrate 10, and a reduction lens 4 for reducing the laser beam passing through the mask 3 to a smaller size. The laser beam generator 1 generally generates a light beam with a wavelength of about 308 nanometers (nm) using XeCl or 248 nanometers (nm) using KrF in an excimer laser. The laser beam generator 1 discharges an untreated laser beam. The discharged laser beam passes through an attenuator (not shown), in which the energy intensity is controlled. The laser beam is then irradiated to the focusing lens 2.
The substrate 10 having an amorphous silicon layer deposited thereon is fixed to an X-Y stage 5, which faces the mask 3. In order to crystallize an entire surface of the substrate 10, the X-Y stage is minutely displaced, thereby gradually expanding the crystallizing region. The mask 3 includes an open part ‘A’ allowing the laser beam to pass through, and a closed part ‘B’ absorbing the laser beam. The width of the open part ‘A’ determines the lateral growth length of the grains formed after the first exposure.
A method for crystallizing silicon using the general SLS device of FIG. 2 according to the related art will be described as follows. FIG. 3 is a cross-sectional view schematically illustrating the laser crystallizing process according to the related art. As shown in FIG. 3, a buffer layer 21 and an amorphous silicon layer 22 are sequentially formed on a substrate 20. Then, a mask (not shown) with sequentially alternating open parts and closed parts is placed over the substrate 20 having the amorphous silicon layer 22 deposited thereon. Thereafter, the laser irradiation process is performed on the amorphous silicon layer 22. The amorphous silicon layer 22 is generally deposited on the substrate 20 by a chemical vapor deposition (CVD) method, which results in a high content of hydrogen within the amorphous silicon layer 22 immediately after the deposition. However, when heated, hydrogen tends to escape from the thin film. Therefore, a primary heat treatment may be carried out on the amorphous silicon layer 22 in order to perform the dehydrogenation process. When the hydrogen is not eliminated during a prior process, the surface of the crystallized silicon layer becomes rough, thereby resulting in the poor electrical characteristics.
Hereinafter, a method for crystallizing the silicon by using the mask having the alternating open and closed parts of the same width in the aforementioned general SLS device will be described as follows. FIG. 4 is a plan view illustrating a related art mask. Referring to FIG. 4, the related art mask includes a plurality of open parts A and closed parts B, wherein the open and closed parts A and B alternate. The width of the open and closed parts A and B may be controlled. In FIG. 4, the width of the open part A is similar to that of the closed part B. Because there may be uncrystallized portions of the amorphous silicon layer left after the primary laser irradiation process, a second laser irradiation process is performed thereto. In the aforementioned silicon crystallization process, the size of crystal particle grown in the X-axis direction or Y-axis direction on the substrate is limited to 0.3 μm to 1 μm. Thus, a silicon crystallization process for producing large-sized crystal particles having great mobility characteristics is proposed by the crystallizing the substrate in the Y-axis direction after completion of the crystallization in the X-axis direction by moving the substrate.
Hereinafter, the silicon crystallization process performed in the X-axis and Y-axis directions will be described as follows. FIG. 5A to FIG. 5E are plan views illustrating a related art method for crystallizing the silicon by using the mask of FIG. 4. In FIG. 5A to FIG. 5E, the arrow designates a direction that the crystallization proceeds. Also, a substrate 30 is moved along the X-axis or the Y-axis by moving a stage having the substrate 30 placed thereon.
As shown in FIG. 5A, a mask 25 is placed above the substrate 30, and one laser pulse is irradiated thereon, whereby crystallation regions are formed on a first crystallizing block H of the substrate 30 depending upon the number of open parts A of the mask 25. In this case, each first crystallizing block H has a width ‘L’ and a length ‘S’. Generally, a laser beam irradiation device places a reduction lens below the mask 25, whereby a laser beam pattern irradiated on the substrate 30 through the mask 25 is reduced by a constant rate of the reduction lens. For example, if the reduction rate is ‘5’, as shown in FIG. 4, and a length and a width of the open part A are set as ‘l ’ and ‘s’, one part on the crystallizing block H irradiated with the laser beam through the open part A of the mask has a length of l/5 and a width of s/5. As one laser pulse is irradiated onto the substrate through the mask 25, the crystallization process is carried out with the first crystallizing block H having the width ‘L’ and the length ‘S’.
As shown in FIG. 5A, after arranging the mask 25 corresponding to the substrate 30, the substrate 30 is moved along the X-axis direction to the right a distance corresponding to the width L of the first crystallizing block H ({circle around (1)}), thereby carrying out the crystallization on a number of lines corresponding to the number of the open parts A of the mask 25.
As shown in FIG. 5B, after completing the crystallization for the open parts A along the X-axis direction to the right, the substrate 30 is moved upward along the Y-axis direction a distance corresponding to a width ‘t’ of one part in the first crystallizing block H ({circle around (2)}). Subsequently, the substrate 30 is moved along the X-axis direction to the left a distance corresponding to the width L of the first crystallizing block H ({circle around (3)}), whereby the crystallization proceeds on the substrate in areas corresponding to the number of the open parts A of the mask 25. Upon completion of the crystallization along a horizontal direction of the substrate 30, a crystallization using the open parts A along a vertical direction of the substrate 30 a distance corresponding to (S+t) is carried out. Subsequently, after moving the substrate 30 upward along the Y-axis direction at a range corresponding to the length S of the first crystallizing block H, the process of ({circle around (1)}) to ({circle around (3)}) is repetitively performed to completely crystallize the entire surface of the substrate on the X-axis direction.
Referring to FIG. 5C, when the crystallization in the X-axis direction on the substrate 30 is completed, the substrate 30 is rotated at 90°. Then, as shown in FIG. 5D, after arranging the mask 25 corresponding to the substrate 30, the substrate 30 is moved along the X-axis direction towards the right a distance corresponding to the width L of the first crystallizing block H, to carry out the crystallization on a number of lines corresponding to the number of the open parts A of the mask 25. When the substrate is rotated at 90° during the crystallization process such as explained in FIG. 5A, the grains primarily grown in one direction on the substrate act as seeds to further crystal growth in a direction perpendicular to the previous crystals, thereby increasing the size of the grains.
After completing the crystallization in the X-axis direction to the right corresponding to the open parts A along a horizontal direction of the substrate 30 (which corresponds to the vertical direction of the substrate 30 in FIG. 5A), as shown in FIG. 5E, the substrate 30 is moved upward along the Y-axis direction a distance corresponding to the width ‘t’ of one part in the first crystallizing block H ({circle around (6)}). Then, the substrate 30 is moved along the X-axis direction to the left a distance corresponding to the width L of the first crystallizing block H ({circle around (7)}), to carry out the crystallization on a number of lines corresponding to the number of the open parts A of the mask 25. Accordingly, when the crystallization in the X-axis direction to the left is completed along the horizontal direction of the substrate 30, the crystallization process continues after moving the substrate 30 vertically a distance corresponding to (S+t). After the substrate 30 is moved upward along the Y-axis direction a distance corresponding to the length S of the first crystallizing block H, the aforementioned process including ({circle around (6)}) is repetitively performed to complete the crystallization on the entire surface of the substrate 30 perpendicular to the first crystallization direction.
Generally, the crystallization process is carried out where the mask is in a fixed state, and the substrate 30 is moved for crystallization. At this time, the substrate 30 is moved in a direction opposite to the direction of the crystallization (the arrow direction). That is, in case of ({circle around (1)}), the crystallization progresses on the substrate 30 through the mask as the substrate 30 is moved along the X-axis direction to the right a distance corresponding to the beam length L of the open part A. During the process of ({circle around (2)}), the substrate 30 is moved upward along the Y-axis direction a distance corresponding to half (½) of the total width of the open and closed parts ((a+b)/2), and then again moved along the X-axis direction to the left a distance corresponding to the beam length L of the open part A, whereby the crystallization proceeds on the substrate 30 through the mask. Accordingly, after the crystallization on the substrate 30 corresponding to the mask along the X-axis direction through the movement of ({circle around (1)}) to ({circle around (3)}), the substrate 30 is moved upward along the Y-axis direction at the range corresponding to the length of the mask, so that the crystallization is progressed again through the process of ({circle around (1)}) to ({circle around (3)}).
However, the related art laser crystallization device and the method for crystallizing the silicon by using the same have the following disadvantages.
In the process of the X-axis and the Y-axis directions for improving the device characteristics, the crystallization on the substrate is progressed along the X-axis direction with the laser crystallization device including the mask having the open parts of the same shape, and then the substrate is rotated at 90° for crystallization in the perpendicular direction. However, if the substrate is large, it is difficult to rotate the substrate. Also, the process of rotating the substrate takes long time, thereby increasing the time for crystallizing the silicon on the substrate.